1. Field of the Invention
The present invention relates to an optical disk used as a medium for recording information and also to an optical disk apparatus for recording/reproducing information on/from the optical disk.
2. Description of Related Art
As a conventional technique for detecting an address signal on a signal surface of an optical disk, the technique described in “DVD Specifications for DVD-R for General, Physical Specifications Version 2.0” is known, for example. This conventional technique will be described with reference to FIGS. 10A to 10D, FIG. 11, and FIG. 12, which schematically show this technique.
FIG. 10A shows the basic configuration of a conventional optical disk apparatus. FIG. 10B is a plan view showing the state where a light spot scans a signal surface of an optical disk. FIG. 10C is a plan view showing one form of a light spot detected on a photodetector. FIG. 10D is a plan view showing another form of a light spot detected on the photodetector.
In FIG. 10A, light emitted from a light source 1 passes through a beam splitter 2 and then is converted into parallel light by a collimator lens 3. Then, an objective lens 4 focuses the parallel light on an optical disk signal surface (hereinafter referred to simply as a “signal surface”) 5S of an optical disk substrate 5, whereby the parallel light is focused into converging light 6. Light reflected by the signal surface 5S is converted into parallel light by the objective lens 4. The parallel light is converged by the collimator lens 3 and then reflected by a split surface 2a of the beam splitter 2 to be focused on a photodetector 7, whereby the parallel light is focused into converging light 8.
As shown in FIG. 10B, a focal spot 6S on the signal surface 5S is controlled so that it is positioned on one of guide grooves 5G, which are recessed portions formed periodically on the signal surface 5S, and scans the guide grooves 5G as the optical disk substrate 5 rotates. On the other hand, as shown in FIG. 10C, a detection surface of the photodetector 7 is divided into four photodetecting elements 7a, 7b, 7c, and 7d by parting lines 71 and 72 that are orthogonal to each other. Accordingly, a focal spot on the detection surface also is divided into four focal spots 8a, 8b, 8c, and 8d by the parting lines 71 and 72. The direction in which the parting line 71 extends corresponds to the direction in which the guide grooves 5G extend on the signal surface 5S. A tracking error signal (TE signal) that indicates an error in tracking the guide grooves 5G, a differential phase detection signal (DPD signal) that indicates an error in tracking a pit, a recording mark, or the like, and a reproduction signal (RF signal) are generated based on the following (Formula 1), (Formula 2), and (Formula 3), respectively, where 7A, 7B, 7C, and 7D represent detection signals detected at the photodetecting elements 7a, 7b, 7c, and 7d, respectively.TE=(7A+7D)−(7B+7C)  (Formula 1)DPD=(7A+7C)−(7B+7D)  (Formula 2)RF=7A+7B+7C+7D  (Formula 3)
The optical disk apparatus may be configured so that a hologram 9 further is disposed between the beam splitter 2 and the photodetector 7. In this case, due to the diffraction by the hologram 9, the focal spot is divided into four focal spots as shown in FIG. 10D, namely, a focal spot 8a′ that is formed within the photodetecting element 7a, a focal spot 8b′ that is formed within the photodetecting element 7b, a focal spot 8c′ that is formed within the photodetecting element 7c, and a focal spot 8d′ that is formed within the photodetecting element 7d. 
In the conventional optical disk apparatus, an address signal of the optical disk is detected based on the TE signal regardless of the presence or absence of the hologram 9.
FIG. 11 is a plan view showing the shape of an address signal pre-pit 5A of a conventional optical disk. In FIG. 11, a portion between each pair of adjacent guide grooves 5G is a land portion 5L. The address signal pre-pit 5A has the same height as the guide groove 5G and protrudes from the guide groove 5G toward the region of the land portion 5L on one side, with the amount of protrusion being “a” and the length of the protruding portion being “b”. The amount of protrusion “a” is a distance measured from the center line GC of the guide groove 5G, and the length “b” of the protruding portion is a distance along the direction of the guide groove 5G. An address assigned to each location on the signal surface 5S is indicated by the arrangement of the pre-pit 5A on the signal surface 5S in accordance with a predetermined rule. When the focal spot 6S scans the guide grooves 5G, the TE signal is affected by the presence of the address signal pre-pit 5A.
An example thereof is shown in FIG. 12. FIG. 12 shows a TE signal waveform 10a and a DPD signal waveform 11a obtained at a portion around the address signal pre-pit 5A when reproduction is performed with respect to an unrecorded region. In FIG. 12, the horizontal axis represents a position (μm) in the length direction of the guide groove 5G and the vertical axis represents an amount of signal (%). Note here that FIG. 12 is directed to an example in which the measurement conditions are as follows: the light source 1 has a wavelength λ of 0.66 μm, the objective lens 4 has a NA of 0.62, the guide grooves 5G are arranged at a pitch p of 0.74 μm, the guide grooves 5G and the pre-pit 5A have an optical depth d of 7 p/72, the guide grooves 5G have a width w of 0.30 μm, the amount of protrusion “a” is 0.23 μm, and the length “b” of the protruding portion is 4T, where T represents a length corresponding to a clock frequency and is set to 0.133 μm. As shown in FIG. 12, a large amplitude is caused in the TE signal waveform 10a by the address signal pre-pit 5A. By detecting this amplitude waveform through the comparison with a predetermined slice level, it is possible to detect an address signal. It is to be noted here that, as shown in FIG. 12, the DPD signal waveform 11a is not affected by the pre-pit 5A and thus shows substantially no amplitude.
In the conventional optical disk and optical disk apparatus as described above, there has been a problem as follows. That is, changes in the TE signal waveform and the DPD signal waveform obtained at a portion around the address signal pre-pit 5A when reproduction is performed with respect to a recorded region are perceived as a problem. FIG. 13 shows a TE signal waveform 10b and a DPD signal waveform 11b obtained at a portion around the address signal pre-pit 5A in a recorded state. FIG. 13 shows the state where the optical depth d of the guide groove 5G and the pre-pit 5A becomes p/4 by recording. As clear from FIG. 13, the TE signal waveform 10b obtained after recording has substantially no amplitude regardless of the presence of the pre-pit 5A, so that it is not possible to detect the amplitude waveform.
This phenomenon can be explained as follows with reference to FIG. 14. FIG. 14 shows the relationship between a detection signal amplitude and an optical depth of the guide groove 5G and the pre-pit 5A in an optical disk of a disk format such as DVD-R or DVD-RW. In FIG. 14, the horizontal axis represents an optical depth (λ/720) and the vertical axis represents amplitude (%) of a detection signal. Note here that the measurement conditions are as follows: the light source 1 has a wavelength λ of 0.66 μm, the objective lens 4 has a NA of 0.62, the guide grooves 5G are arranged at a pitch p of 0.74 μm, and the guide grooves 5G have a width w of 0.30 μm. A TE signal amplitude 12 denotes the value of the total amplitude detected when the focal spot 6S traverses the guide groove 5G. A RF signal amplitude 13 denotes the value of signal amplitude detected when a 3T continuous recording signal in an isolated track is reproduced. A DPD signal amplitude 14 denotes the value of signal amplitude detected when a 4T continuous phase pit signal is reproduced with an amount of off-track being 0.1 μm.
As shown in FIG. 14, the TE signal amplitude 12 reaches its maximum when the optical depth d is p/8 and its minimum (zero) when the optical depth d is p/4. On the other hand, the RF signal amplitude 13 and the DPD signal amplitude 14 reach their maximum when the optical depth d is p/4. In the optical disk of a disk format such as DVD-R or DVD-RW, the guide groove 5G in an unrecorded state has an optical depth d of about λ/20 to λ/10, and a signal mark formed by recording serves as a phase pit having an optical depth d of about λ/8 to λ/5. Accordingly, when combined with the guide groove 5G that is originally present on the optical disk, the recording mark has an optical depth d in the vicinity of p/4, thereby allowing the RF signal amplitude to reach its maximum. On the other hand, when the recording mark is formed so as to lie over the pre-pit 5A, the pre-pit 5A causes no amplitude in the TE signal, so that the reading of an address signal after recording (also during recording) becomes impossible. In particular, in the case of high-speed recording in an optical disk of a disk format such as DVD-R, the recording mark is liable to expand, which may cause the recording mark to cover the pre-pit 5A entirely, thereby further inhibiting the generation of amplitude in the TE signal.